The present invention relates to a method intended to allow the homogenizing of electrical signals generated by a detection system and transmitted to an exploiting device of the multiplexer type, for example, a charge-coupled device (CCD). It applies more particularly to the processing signals generated by an array of photodetectors such as photodiodes whose photosides have a solid angle of view different from one site to another. Even more specifically, this invention applies to infrared detectors which, provided with their readout circuits, are assembled in a cryogenic enclosure.
An exploiting or readout device or readout circuit is known in the art to mean a circuit for reading and time-shifted processing (multiplexing) of the various signals passing through this circuit.
It is well understood that the electro-optical performance of photodetectors, and particularly infra-red detectors, is improved by limiting their useful solid angle, also called the field of view. This limitation is most often carried out by diaphragms having an opening adapted to the spatial structure of the assembly formed by the detectors. It is also possible to use an optical system called an "aperture transfer" system, but this type of device is complex and expensive.
The use of such detection systems provided with a diaphragm, in particular for assemblies of large size, whether they are linear arrays or matrices, creates viewing aspects of the opening of the diaphragm by the photosites at an angle of view which is different from one photosite to another, the latter reducing with the distance of the photosite from the optical axis of the system. The solid angle of view or field of view of each of the photosites can be broken down into two separate solid angles, one called the useful angle of view or the "signal" angle of view enclosing the optical system preceding the diaphragm and the other called the "parasitic" solid angle of view containing the radiation limited by the diaphragm which cannot pass through the optical system and is reflected by the hot and cold parts of the enclosure of the dewar, of the diaphragm or of the detectors, or emitted by the hot parts of the dewar. In this way, for an observed homogeneous background, the photon flux reaching each detector is fluctuating and varies with the position of the latter, as this flux is proportional to the solid angle or field of view. This inconsistency affects either the signal field of view and the parasitic field of view or, more particularly, the parasitic field of view in the case of special optical systems. In fact, the continuous response of the system is made inconsistent by it and it harms the functioning of the system. In other words, the response of the system observing a homogeneous background is not itself homogeneous.
It has been proposed in French published patent application 2,613,831 to limit or even to eliminate this source of inconsistency by varying the surface area of the photosites in such a way that the product of the solid angle of view and the area of each of the photodetectors is constant, thus representing the constancy of the photon flux for all of the photosites at the level of the detection. However, these photodetectors having different surface areas have different electrical properties, particularly when these detectors are photodiodes. In fact, the inconsistency is removed at the level of the detection. On the other hand, the electrical and spatial characteristics are not constant from one photodetector to another. In this way, even if the problem of homogeneity is solved at the level of the detection, this problem is transferred to the exploitation level. The problem has in fact only been shifted. For example, in the case of photodiodes, the reverse dynamic resistance, the short-circuit current, the saturation current and the original resistance of each of the diodes vary as a function of the surface area of the latter resulting in an inconsistent functioning of the readout circuit. The use of such a structure therefore becomes very complex.
In order to improve the results of an imaging device, it is known to induce at the level of the exploiting circuit either a division, also called partitioning, of the created charges or a lowering, also called skimming, of the latter in order to use only a more reduced number of charges for the purpose of eliminating, or of reducing the background noise. This type of device is for example, described in European Patent documents A-0-148,086 and A-0,251,386. However, this type of device is still of limited use, taking account of the fact that the processes are performed independently from the photon flux dispersions. In fact, in the European Patent document 0,251,386 for example, implementation assumes the use of a reference charge generator capable of simulating a homogeneous background. There is therefore no correlation between the incident photon flux and the division and/or lowering.
There has also been described in the publication by Chow et al in IEEE TRANSACTIONS ON ELECTRON DEVICES (vol. ED 29, No 1, January 1982, pages 3-13), a conventional input stage of a CCD provided with a lowering function and a division function, and an input stage with a feed-back amplifier intended to compensate for the detection diodes exhibiting a low reverse resistance. Once again, the signal supplied and analyzed takes no account of the incident photon flux and cannot therefore validly modulate this signal as a function of the positioning of the detection circuit.
These techniques are described for exploiting circuits of the CCD type, which carry out a current/charge conversion of the current from the photosites. These techniques are also practiced, without modifying the analysis thereof, for exploiting circuits carrying out a current/voltage conversion of the current from the photosites.